Eukaryotic elongation factor 2 kinase (eEF2K) critically regulates translation elongation by controlling the activity of eEF2, which catalyses the translocation reaction of the ribosome. eEF2K phosphorylates eEF2 and prevents its binding to the ribosome to inhibit translation elongation. eEF2K is activated by elevated Ca2+ levels via calmodulin (CaM), although the molecular mechanism of activation is not understood. A conserved region at the N-terminus of eEF2K has been shown to be a key interaction site for CaM. A peptide corresponding to the CaM binding region of eEF2K (eEF2K82-100) as well as longer eEF2K fragments to represent the full-length protein have been studied to dissect the molecular mechanism of eEF2K activation by Ca2+/CaM. Nuclear magnetic resonance (NMR) spectroscopy has been used to determine the binding site and the mechanism of peptide recognition and the three-dimensional solution structure of the Ca2+/CaM: eEF2K82-100 complex has been elucidated. These investigations revealed important roles for particular residues within the CaM binding region of eEF2K and in CaM itself and also resulted in a model for eEF2K activation that involves residues in the kinase domain, as well as the CaM binding region. The elucidation of this protein complex structure provides a vital tool in the study of eEF2K function, critical in cancer biology.